MANUFACTURED AGGREGATE AS IMPORTED
BACKFILL MATERIAL FOR PLASTIC PIPES
D.J. Newport, Lecturer, University of East London UK
D.C. Wijeyesekara, Reader, University of East London UK
K. Edwards, Special Subject Tutor, University of East London UK
Abstract: Research work undertaken at the University of East London has
produced new forms of sustainable manufactured aggregate, which gives a backfill
material which has many advantages over natural materials. The unique
geotechnical properties of the aggregate make it an idea material for the laying of
plastic pipes allowing better support and stability, contributing to the longevity of
pipelines.
INTRODUCTION
The safe installation and long term performance of plastic pipe systems is now
recognised as partially a function of the excavation and reinstatement techniques
adopted by the installer. Historic evidence has shown that poor laying technique has
led to premature failure of pipe systems. Many failures have been shown to have
arisen because of stress point, being created by flints or other angular hard material
within the trench.
Plastic pipes generally fall within the classification of being “flexible”, deriving much
of their support from passive soil pressure. This passive pressure is a complex action
between the pipe, fill material and natural soil structure. The construction of the
excavation depth, width and backfill material all influence the performance of the
finished pipeline.
In smaller diameter systems, jointing of the pipe often occurs at ground level, then
significant lengths of pipe can be installed without the need for a man entry trench.
This practice allows for rapid, economical and safe construction. Under these
circumstances the pipe is normally laid on a 100mm bed of grannular material and
backfilled to 100mm above the crown of the pipe with the same material. The trench
must be filled evenly either side of the pipe and adequately compacted to give the
necessary support. The narrower the trench the more difficult it is to ensure that
adequate compaction is achieved. With flexible pipe systems, the deflection under
vertical load is reduced by the resistance obtained from the sidefill, which will pass
the load onto the trench wall. Ensuring that these loads are evenly transmitted is
important to prevent local over stressing of areas of the native trench wall.
A great deal of work has been undertaken to assess the best specification for the laying
of underground plastic pipe systems. In the UK the Water Industry Sewers and Water
Mains Committee have published information to assist in the structural design of
buried plastic pipe systems. However the successful installation of plastic pipe
systems requires a great deal of care in the placing and compaction of backfill which
necessitates competent and dedicated supervision and quality control. One of the
crucial decisions required to ensure effective construction is the choice of bed and
surround material. As experience has shown that the wrong material can led to
pipeline failure, many installers are using imported granular materials to prevent pipe
damage or poor compaction. The consequence of this use of imported material is
significant in terms of the need for either quarried or sea dredged materal. This
requirement impacts on the environmental costs of transportation and disposal of
waste material.
AGGREGATE IN THE UK
The UK Government has for sometime been concerned about the sustainability of
mineral extraction and carefully monitor the amount of aggregate that is used and its
sources. Table 1 shows the likely quantity of aggregate to be used in England until the
year 2016. The table shows how alternative materials are becoming a very significant
contributor to the overall aggregate requirments.
Table 1: National And Regional Guidelines For Aggregates Provision In England,
2001 - 2016 (Million Tonnes)1
Guidelines for land-won
production
Land-won Land-won
New Regions Sand &
Crushed
Gravel
Rock
South East
212
35
England
19
0
London
East of
256
8
England
East
165
523
Midlands
West
162
93
Midlands
106
453
South West
55
167
North West
Yorkshire &
73
220
the Humber
20
119
North East
England
1068
1618
Assumptions
Marine
Sand &
Gravel
Alternative Net Imports
Materials
to England
120
118
85
53
82
6
32
110
8
0
95
0
0
88
16
9
4
121
101
4
50
3
128
0
9
230
76
919
0
169
The UK Government is also acutely aware of how the aggregate extraction industry is
viewed by the public. It has therefore undertaken research to assess how much the public
are willing to pay to reduce significantly the enviromental effects of quarrying. An
extract from that report 2 is as follows:
“The research was mainly undertaken to inform decisions about the regulation and
possible taxation of the aggregates industry.
The key results are that, on average, after grossing up from survey results to the whole
relevant population:



People living near hard rock quarries expressed a willingness to pay of about £10
per year, which is equivalent to around £0.35 per tonne of crushed rock
produced.
People living near sand and gravel operations expressed a willingness to pay of
about £15 per year, which is equivalent to around £2 per tonne of sand and
gravel produced.
Households throughout Great Britain expressed a willingness to pay of about £5
per year to stop the environmental effects of quarries located in National Parks,
which is equivalent to around £10 per tonne of aggregates produced in National
Parks.
These estimates have been consistently designed to be on the conservative, or low, side.”
This research show that the general public are willing to pay to reduce the
environmental impact of mineral extractions and therefore the government are
considering how taxation can be used to encourage the development of sustainable
alternatives.
In a similar vane the UK Government 3 has taken the following view in respect to the
recycling of waste materials arising from the construction industry and has the
following comments:



Around 70 million tonnes of demolition and construction waste, and almost 100
million tonnes of mining and quarrying waste are generated each year. Although
there is considerable potential for using these wastes as aggregates, large quantities
either remain on site or are landfilled.
Government policy aims to minimise the loss of land and the environmental impacts
associated with primary extraction and waste disposal by increasing the amount of
recycled and secondary materials used in construction. The aim is to increase their
use from about 30 million tonnes in 1989 to about 55 million tonnes per year by
2006. Therefore, there is a need to provide locations for both new and expanding
operations.
Due to local environmental effects and a traditionally poor image, recycled and
secondary aggregates operations are often regarded as 'unneighbourly'. Planning
permission is often refused because the local environmental effects are considered
to be greater than the environmental sustainability benefits of the development.
The Guidance addresses this issue by outlining the siting considerations,
management controls and physical measures to avoid or reduce the local
environmental impacts of producing recycled and secondary aggregates.

All these elements have led the University of East London in the UK to examine how
a new form of artificial aggregate can be manufactured which is both sustainable and
gives better geotechnical properties. This new form of aggregate has substantial
benefits for use in the construction of pipelines.
INTRODUCTION TO THE MANUFACTURING PROCESS
The Pilot Plant is situated at the Manufactured Aggregate Research Centre (MARC),
University of East London and has been designed using state-of-the-art technology.
The incorporation of laboratory scale equipment and the replication of a full-scale
production plant has enabled research and development to be undertaken from initial
research to pilot scale production prior to commercial development. The facility
produces aggregates within defined requirements in sufficient quantities for
representative testing.
Manufactured aggregates have been produced for more than 70 years from such
materials as clay, shale, slate and slag, and more recently from pulverised-fuel ash
(PFA). This facility enables large-scale feasibility testing to be carried out on
aggregates designed and manufactured using a wide variety of waste materials.
Current developments in the science and technology of thermal processing now enable
aggregates to be designed and manufactured from various combinations of resource
materials, such as:





residues from bio-degradable materials
municipal solid wastes
industrial and by-product ashes
reclaimed argillaceous and granular materials
demolition and construction-derived wastes
The pilot plant process allows waste materials to be used in the manufacture of
commercially viable aggregates. Many waste materials contain combustible
components, which provide some of the energy required for kilning, and reduce the
density of the finished product, so improving the overall efficiency of the process.
Other wastes contain materials that can be expanded by heat (bloating) to reduce
density. This process can transform wastes, which are difficult or extremely costly to
manage, into useful, safe and valuable aggregates.
In order to satisfy European construction product specifications, the MARC facility
has been designed to manufacture aggregates for high-performance applications
especially to satisfy legislation for ecotoxicity. The processing technology enables
aggregate to be designed and produced to meet specific market applications thus
allowing natural aggregates to be substituted by manufactured aggregates of superior
technical performance at a competitive cost. This project aims to significantly reduce
the disposal of both hazardous and non-hazardous wastes to landfill. By using
materials perceived as wastes as resources for aggregate manufacture, this project has
demonstrated a sustainable waste management policy in support of Waste Strategy
2000 by:




reducing waste to landfill and developing new waste management techniques
conserving non-renewable resources
conserving the natural environment
lowering construction costs
DESIGN OF FACILITIES
The facilities provided by this project consist of:
1. A portable state-of-the-art integrated bench top mixer, pelletiser and Trefoil
kiln based at MARC for feasibility studies including initial raw material
assessment and mix optimisation.
2. A static pilot plant at MARC which can be operated in separate stages or
continuously
Figure 1 Digrammatic view of the Pilot scale plant for aggregate manufacture
The gas-fired high-temperature (over 11000C) rotary Trefoil kiln is at the heart of the
facility. The name Trefoil relates to the internal shape of the kiln, which unlike
conventional cylindrical kilns, is similar to a three-leaf clover. This shape, when
rotated, allows a bed of pellets to gently cascade from leaf to leaf with every
revolution of the kiln to ensure even distribution and mixing. The super alloy steel
lining of the kiln wall is shaped to allow a greater proportion of the radiant heat
emitted from the wall to be directed back to the bed of pellets resulting in an even
temperature throughout and more efficient heating. Insulating fibres surround the kiln
wall to further minimise heat loss. The resulting low thermal mass gives this
lightweight kiln excellent temperature control permitting rapid heat-up and cool-down
times. This is not possible with traditional kilns lined with refractory bricks.
The following seven stages are integrated within the pilot plant:
BLENDING AND METERING
Up to three dry powdered resource materials can be proportioned by mass within this
stage using three metering screws, each having a variable speed drive for calibration
with any suitable material.
MIXING
The metering screws discharge the resource materials into a sealed high-speed mixer.
The mixer incorporates a shaft controlled to rotate up to 1450 rpm. The shaft is fitted
with paddles that can be adjusted to control the powder retention time within the
mixer to ensure complete homogenisation. The mixer discharges the finished blend to
a screw, which transfers the material to the pan pelletiser.
PELLETISING
The homogenised raw material is fed at a predetermined rate into a pan pelletiser and
sprayed simultaneously with a prescribed amount of pressurised water through
atomising nozzles. The pelletiser consists of a rotating inclined pan with controllable
angle of inclination and rotational speed. The combination of rotating action,
inclination, raw material blend, water content and retention time causes the powder to
agglomerate to form pellets of the required size and density.
GRADING
After pelletising, pellets not conforming to the required size are removed by an
inclined vibrating screen, fitted with chutes attached to collection bins for
reprocessing. The 'green' pellets of the correct size are fed by conveyor to the
coater/drier.
COATING/DRYING
The moisture content of the 'green' pellets must be reduced prior to high temperature
kilning to prevent them disintegrating by steam under pressure. The 'green' pellets can
also be coated with a prescribed quantity of powdered coating material to enhance
their performance. Moisture is removed by rotating the pellets at about 1500C in the
inclined gas-fired cylindrical drier, which also gives the pellets a hard outer shell, and
coating to increase their strength for high temperature kilning.
HIGH TEMPERATURE TREFOIL KILNING
Pellets are fed into the Trefoil kiln at a controlled rate by a vibrating feeder. The kiln
can operate continuously for periods of at least 10 hours at the required temperature
and residence time, the latter being controlled by varying the speed of rotation and
pellet feed rate. At a sufficiently high temperature any combustible constituents within
the pellets are burnt and any expandable materials are 'bloated'. Both actions reduce
pellet density. The high temperature melts the glassy constituents to form a vitrified
interior and exterior structure thereby reducing the penetrability of the finished pellets.
The precise temperature control required during kilning is achieved by controlling the
gas burner via a non-contact thermometer continuously monitoring the temperature of
the pellet bed. This avoids undesired melting of the pellet surface and consequential
agglomeration. The fired pellets exit the rear of the kiln via a chute for cooling.
COOLING
The fired pellets are cooled rapidly to 'freeze' their structure before storage in sealed
containers.
Typical uses for manufactured aggregates:








Lightweight structural insitu or pre-cast concrete
Lightweight concrete masonry
Land drainage and lightweight fill
Filter media for sewage, oil, etc.
Horticultural and hydroponic media
Aircraft and vehicle arrestor beds
Packaging
Bed and surround for pipeline construction
GEOTECHNICAL PROPERTIES OF MANUFACTURED AGGREGATE
A series of relevant aggregate testing is being carried out at the University of East
London on a wide variety of manufactured aggregates. Table 2 gives a summary of
results from relevant tests carried out on 3 different manufactured aggregates and natural
pea gravel. The three manufactured aggregates reported in this paper represent just a few
possible variations in the type and proportions of raw feed that can be used. The
manufactured aggregates A, B and C have a spherical shape by virtue of the
manufacturing process whereas the flint pea gravel, D is angular. The spherical shape
promotes self-compaction and is an ideal fill for pipe surrounds. Furthermore the
spherical particles pack to give a relatively higher porosity and consequently a lower
density. The firing during manufacture produces a lightweight aggregate with specific
gravities in the order of 1.6 – 1.7, comparably lower to that of the natural aggregate.
Despite the distinct differences in the surface roughness of the natural aggregates and the
manufactured aggregates, the angles of friction observed were very similar. The modulus
of elasticity for the manufactured aggregates was observed to be slightly less but is of the
same order as that of the pea gravel.
Figure 2 shows the particle size distribution curves for the four aggregates. The
proportions in the raw feed and the process parameters can be selected to produce an
aggregate of a desired grading. The manufactured aggregates tested have a uniformity
coefficient marginally larger than that of the natural aggregate and this illustrates the
flexibility that is available in the process. The very similar effective sizes suggest that the
drainage properties of the four aggregates will not be very different from one another.
Particle Size Distribution
100
80
C
70
60
A
50
40
B
30
D
20
10
0
1.2
3.35
4
5
6.3
8
9.5
11.2
14
20
25
Particle size (mm)
FIGURE 2: Particle Size Distribution Curves for the Tested Aggregates
Percentage passing (%)
90
TABLE 2: Summary of Geotechnical Properties of the Aggregates tested
PROPERTY
Description
Colour
Specific gravity
Mean dry density
(kG/m3)
Mean void ratio
A
AGGREGATE SAMPLE TESTED
B
C
D
Dredge Mix:
45% DM, 20%
PFA, 20% ISSA,
10% SS, 5% FA*
Dark brown
Filter Cake:
50% PGW, 45%
CMDW, 5% RG*
Natural Flint pea
gravel
Grey
1.7
1.62
1.64
Various brown
black white
2.55
820
775
695
1440
Standard
Aggregate Mix:
45% OC, 45%
PFA, 10% SS *
Brown
1.082
1.085
1.362
0.772
Mean porosity
(%)
52
52.1
57.7
43.6
Shape
Spherical
Spherical
Spherical
Angular
Effective size
(mm)
6.05
4.92
5.02
7.85
Median size
(mm)
8.85
7.76
9.75
9.74
Uniformity
coefficient
1.55
1.78
2.33
1.33
Coefficient of
curvature
1.05
0.96
0.9
0.97
Aggregate
crushing value
38
54
51
19
(%)
Dry density after
crushing test
2411
1837
1503
6156
(kG/m3)
Strain at 400kN
load (%)
34.4
42.9
46.8
23.4
Durability loss
with dry abrasion
2.7
4.6
3.7
0.2
(%)
Durability loss
after slaking in
1.99
7.87
1.19
0.49
fresh water (%)
Durability loss
after slaking in
1.3
6.74
1.42
0.88
sea water (%)
Angle of friction
38
41
40
39
()
Modulus of
elasticity (MPa)
65
52
48
95
* Abbreviations; CMDW:- Construction material and demolition waste (particulate), DM:- Dredged
material, FA:- Fly ash, ISSA:- Incinerated sewage sludge ash, OC:- overburden clay, PFA:- Pulverised fly
ash, RG:- Recycled glass (particulate), SS:- Sewage sludge.
The aggregate crushing values for the manufactured aggregates are about twice or more
than that of the pea gravel. This reflects the relatively more brittle nature of the
manufactured aggregates. Similar variations can be noticed with the durability of the
aggregates in dry and wet conditions. The highest durability loss was with the aggregate
B, which would have had a larger proportion of fines, or clay sized particles in the
dredged material. The durability values observed do not seem to be adversely affected by
salinity. Overall this analysis shows the performance of the aggregates tested to be
suitable for this application and provides a light, flexibly sized, easily compacted yet
strong material.
A programme of experimental work will continue at MARC to widen the envelope of raw
materials and application available, using this manufacturing technique.
CONCLUSION
The work undertaken at the University of East London has shown that manufactured
artificial aggregates cannot only replace, but is some respects are superior in behaviour to
natural materials. The importance of ensuring that plastic pipelines are properly installed
is paramount to the longevity of the asset. The use of manufactured artificial materials
will not only support effective construction, but will reduce the need to use precious
natural resources and is more sustainable and effective than the use of other recycled
materials.
REFERENCES
1
2
3
Office of the Deputy Prime Minister, National and Regional Guidelines for
Aggregate Provision in England 2001-2016, 2003.
London Economics Report for DETR, Environmental Costs and Benefits of
Supply of Aggregates, 1998.
Office of the Deputy Prime Minister, Controlling Environmental Effects: recycled
and secondary aggregate production, 2003
KEYWORDS
Plastic Pipes, Manufactured Aggregate, Sustainable, Geotechnical, MARC